Thread: ArrayList of ArrayList?

Hello JavaProgrammingForums users!

I come to you with this problem: I am attempting to create an ArrayList of ArrayLists for an assignment, but I'm hitting a roadblock when it comes to figuring out how to populate this construction. So far it seems like I'm able to make an arraylist and get it to print out a sample where all of the values are null, but I cannot figure out how to go further to changing those data values, add another row of data to the arraylist, etc.

Here's what I've gotten so far:

package test;
 
import java.util.ArrayList;
 
 
class SampleTableClass<Type> {
 
    private Type type;
    ArrayList<ArrayList<Type>> array;
 
 
    public void initializeArray()
    {
        array = new ArrayList<ArrayList<Type>>();
    }
 
    public void fillArray( int rows, int columns)
    {
        for (int r=0; r<rows; r++){
            array.add(r, new ArrayList<Type>());
            for (int c=0; c<columns; c++)         
                array.get(r).add(c, type);                
        }
    }
 
    public void print5x5Array(int rows, int columns)
    {
        fillArray(rows, columns);
        Object[] arrayObjects = array.toArray();
        for (int r=0; r<rows; r++){
            for (int c=0; c<columns; c++){
                System.out.println(arrayObjects[r]);
            }
        }
    }
}
 
public class Main {
       public static void main(String[] args){
       SampleTableClass array = new SampleTableClass();
       array.initializeArray();
       array.print5x5Array(1,5);
       }
}

I feel like I've probably made a simple mistake or failed to grasp one of the main concepts entirely, so feel free to tell me I've done everything wrong if I have, if that's what's necessary to fix it.

Thanks!

Here's some code I wrote up in a couple seconds. I used Strings to show you an example without doing 100% of your homework. It's quite chivalrous of me to give you this much, but I'm assuming by your current knowledge that you're here to learn. so please do.

import java.util.ArrayList;
 
public class Main {
 
    private static ArrayList<ArrayList<String>> parent;
 
    public static void main(String[] args) 
    {  
        parent = new ArrayList<ArrayList<String>>();
    }
 
    public static void addChild(String[] childText)
    {
        ArrayList<String> child = new ArrayList<String>();
        for(String text : childText)
        {
            child.add(text);
        }
 
        parent.add(child);
    }
 
    public static void removeChild(int index)
    {
        if(index >= 0 && index < parent.size())
        {
            parent.remove(index);
        }
    }
 
}

Or I can get sniped by a mod. I should have used the search bar to look for that post.

Okay, that helps quite alot, I had already re-examined my code and had seen that I needed to change the structure of how I had everything set up, and this helps some of the pieces fall into place.

One other quick question though- I'm going to be implementing those functions with Object instead of String so that they can be filled up with whatever object that a user would want to do, but because of that I'm unsure how to put the syntax for that in my main called function, whenever I try to code a line such as

parent.addChild(/*This is where I'm having trouble knowing what to put in here to correctly add a child*/);

Any tips how to word this?